Conventionally, in the Space Shuttle and International Space Station (ISS), the Waste Collector System (WCS) has used air flow to entrain waste. Urine is collected in a funnel held in close proximity to the urethra. Fecal material is collected separately by air flow entrainment. For instance, air may enter in a fairly uniform radial direction under a seat of the WCS. Air drag then helps separate the fecal bolus and draw it into a tank or fecal collection bag. Air is separated from the urine and fecal material, treated for odors, and returned to the spacecraft cabin.
Successful capture and ease of use for simultaneous urination and defecation for mixed gender crews have been consistent limitations of the Space Shuttle and ISS WCS. Additionally, these systems are sensitive to crew alignment on the seat and can result in inadvertent fouling of the collection hardware with fecal material.
The ISS WCS shared many features of the Space Shuttle WCS, but modified the fecal collection from a large storage tank to a removable fecal canister. Using plastic lids, a compactor compressed each individual fecal bag to reduce storage volume. After about 21 uses, the canisters are removed, and stored. To improve air flow predictability, a fan was included for the urine separator, and a separate fan was provided for fecal collection. The toilet on the Russian Soyuz vehicle (Soyuz ACY) is smaller than the Space Shuttle WCS and the ISS WCS. It pulls cabin air through the urine funnel and fecal collector before passing through a static separator, where the urine is stored. The saturated air travels through an odor filtration device before the urine and cabin air are separated by a centrifugal unit. The fecal collection bag is designed for a single use, and the urine tank is limited to 10.2 liters. Urine collection is considered acceptable, whereas fecal collection did not receive favorable review.
The Russian Module toilet (SM ACY) for the ISS has provided long-term WCS use since the ISS was first inhabited. It uses air to assist fecal and urine collection. A centrifugal urine separator removes air from the urine stream. The air flows to a charcoal filter prior to reentry back into the cabin. This unit has an air ring under the seat, similar to the two American designs discussed, to control odor and assist in fecal capture. The fecal canister holds one bag that is pushed into a collection tank after each use. This tank can hold about 21 defecations before removing and replacing the canister. From a crew-satisfaction perspective, the WCS is one of the most personal and intimate systems on the spacecraft. Crew consensus is paramount regarding which elements are needed and which would benefit from improvement.